The present invention relates to a process for the electrolysis of an electrolyte such as seawater which comprises the simultaneous production of hydrogen peroxide and hypohalide to efficiently produce an aqueous solution containing both of these chemicals.
Due to concern that air pollution and deterioration of water quality of rivers and lakes caused by industrial and household wastes can have adverse effects on the environment and human body, technical countermeasures for solving these problems are in urgent demand. In the treatment of drinking water, sewage and waste water, it has been the practice to add a chemical such as chlorine to decolor and sterilize the water to be treated and reduce the COD thereof. However, since the addition of a large amount of chlorine causes the production of harmful materials, i.e., environmental hormones (extrinsic incretion disturbing material) and carcinogenic substances, the recent trend is to add less chlorine added.
Further, under some combustion conditions, the incineration of waste can cause the production of carcinogenic substances (dioxins) which affect the ecosystem and thus has been noted as a safety problem. In order to solve the problems of water treatment, the following processes have been proposed as a substitute for the addition of chlorine.
An electrolysis process utilizes clean electrical energy to control chemical reaction on the surface of electrodes, thereby producing hydrogen, oxygen, ozone, hydrogen peroxide, etc. Thus, the material to be treated can be indirectly decomposed. Alternatively, the material to be treated can be adsorbed to the electrodes where it is directly subjected to electrolysis. An electrolysis process has heretofore been utilized for the treatment of waste water.
Hydrogen peroxide is useful as a fundamental chemical indispensable for treatment in the fields of food, medicine, pulp, fiber and semiconductors in addition to water treatment. In particular, noted future uses include the cleaning of electronic parts and sterilization of medical equipment and facilities. At present, hydrogen peroxide is produced in large amount by an anthraquinone process. The electrolytic production of hydrogen peroxide making best use of the advantage of the electrolysis process has been studied and commercially practiced.
In the electrolysis of water, the cathode reaction normally involves the production of hydrogen. When oxygen is present in the cathode chamber, the reduction of oxygen preferentially proceeds. Thus, by controlling chemical reaction on the surface of the cathode, hydrogen peroxide is produced. This electrolysis process allows on-site production of hydrogen peroxide, eliminating the danger in prolonged storage with a stabilizer and transportation or the necessity of anti-pollution measures. With respect to the on-site electrolytic production of hydrogen peroxide, if seawater is used as an electrolyte, seawater containing hydrogen peroxide is obtained.
Referring to the production of hydrogen peroxide by electrolysis, various electrolytic processes are described for comparison in Journal of Applied Electrochemistry, Vol. 25, 613-(1995). All these processes allow for efficient production of hydrogen peroxide in an atmosphere of an alkaline aqueous solution and thus require the supply of an alkaline component as a starting material. Thus, an aqueous solution of an alkali such as KOH and NaOH is essential. The decomposition of formaldehyde by hydrogen peroxide is described in Journal of the Electrochemical Society, Vol. 140, 1,632-(1993). Journal of the Electrochemical Society, Vol. 141, 174-(1994), proposes a method which comprises electrolysis of purified water as a starting material using an ion exchange membrane wherein ozone and hydrogen peroxide are synthesized at the anode and the cathode, respectively. However, this method has a low current efficiency and thus is not practical. It has been reported that a similar method can be effected under high pressure to raise the current efficiency. However, this proposal, too, is not practical from the standpoint of safety. An electrolysis process using a palladium foil has also been proposed. However, this electrolysis process is limited in its use because it can produce hydrogen peroxide only in a low concentration and adds to cost.
In the conventional electrolysis of seawater, the production of THM (trihalomethanes) from organic material which is unavoidably present in seawater restricts the use of the resulting seawater having a sterilizing effect and has an adverse effect on the environment.
It is known that when an aqueous solution containing halide ion such as seawater is subjected to electrolysis as an anolyte, a hypohalide is produced. This seawater containing hypohalide has a strong sterilizing effect and thus is used to kill bacteria in the seawater.
On the other hand, it has been reported that when hypohalide ion is added to seawater containing hydrogen peroxide, the oxidizing power of the seawater is enhanced (Japanese Patent Laid-Open No. 1996-24870). It is disclosed in the above cited patent publication that when effective chlorine such as hypohalide is added to seawater having a hydrogen peroxide content of 0.01 mg/l to an extent such that no trihalomethane (THM) is produced, seawater having a sterilizing power is produced. However, since hydrogen peroxide, which is a starting material, must be transported from the production site to a remote site where it is used, problems of safety, etc., remain.
It is therefore an object of the invention to provide an electrolytic cell and process for the simultaneous production of hydrogen peroxide and hypochlorous ion, which have heretofore been produced separately.
The foregoing objects of the invention will become apparent from the following detailed description and Examples.
The invention provides an electrolytic cell and process for the production of hydrogen peroxide solution and hypohalide by electrolysis which comprises an anode chamber housing an insoluble anode capable of oxidizing halide ion, a cathode chamber housing a gas diffusion cathode capable of oxidizing an oxygen-containing gas to produce hydrogen peroxide, a membrane separating the anode and cathode chambers, and means for supplying water containing halide ion to the anode chamber and oxygen-containing gas and an electrolyte to the cathode chamber, whereby hypohalide and hydrogen peroxide are produced in the anode chamber and the cathode chamber, respectively. The electrolyte, particularly anolyte supplied to the electrolytic cell, is preferably seawater which has previously been freed of organic material. Further, the catholyte containing hydrogen peroxide thus obtained and the anolyte containing hypochlorous ion may be added to water to be treated upstream and downstream of the electrolytic cell, respectively, making it possible to efficiently treat the water.